Instrumental Measurements with Carbon Paste Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 142

A.similar.situation.occurs.in.the.case.of.carbon.pastes.(or,.liquid.membranes).containing.extract-
able.ion.pairs,.formed.according.to.the.overall.reaction

+ −

Q w ( ) + X w QX o ( ) = ( ) . (6.35)

.

The.principle.for.design.is.as.follows.[1972]:.to.build.an.electrode.responsive.to.anion. X − ,.for.exam-
ple,. the. salt. Q + X − . should. be. incorporated. into. a. nonvolatile. solvent. using. as. a. membrane. plasti-
cizer.or,.in.the.case.of.carbon.pastes,.as.pasting.liquid,.and.the. Q + .ion.must.be.highly.lipophilic..
Similarly.for.an.electrode.responsive.to.cation. Q + ,.an.oil-soluble.salt. Q + X − .is.used,.where.the. X − .ion.
is. lipophilic.. Thus,. the. quaternary. log-chain. alkyl. and. aryl. ammonium. salts. and. high. molecular.
weight.cationic.dyes,.etc.,.are.known.to.behave.as.liquid.anion.exchangers.suitable.for.preparation.
of. anion-selective. electrodes.. Tetraphenylborate. and. high. molecular. anionic. surfactants. such. as.
dodecylsulphate,. etc.,. show. good. selectivity. for. heavier. univalent. inorganic. cations. and. are. also.
used.in.electrodes.for.other.“onium”.ions.
If.such.an.ion-pair.compound.(for.simplicity,.both.moieties.are.univalent).is.dissolved.in.organic.
phase,.one.has.an.electrode.(and.a.potentiometric.cell).responding.to.both.cationic.and.anionic.species:

+ −

E = Constant + 0 059 . log[ Q ] = Constant − 0 059 . log[ X ] . (6.36)

.

w w

In.contrast.to.paste.electrodes.used.for.electrolytic.accumulations,.the.potentiometric.sensors.may.
be.prepared.even.with.the.use.of.polar.liquids.containing.reducible.groups..Thus,.for.example,.nitro-
benzene.extracts.of.ion.pairs.such.as.Ni(phen) 3 (ClO 4 ) 2 .or.Fe(phen) 3 (BF 4 ) 2 .(where.phen.=.1,10-phen-
anthroline). were. reported. as. pasting. liquids. and. ion. exchangers. for. perchlorate. or. luoroborate.
ion-selective.CPEs.[709].
In.Figure.6.9,.a.set.of.potentiometric.curves.is.shown.(in.the.upper.part),.together.with.the.fol-
lowing. corresponding. plot,. both. concerning. the. automated. ion-pair. formation-based. titrations. of.
the.homologous.series.of.quaternary.ammonium.salts.that.varied.in.the.length.of.their.alkyl.chains..
The.titrant.containing.lipophilic.counter.ion.was.tetraphenylborate.(used.as.NaBPh 4 ,.where.Ph.=.
C 6 H 5 .[1975]).
According.to.this,.the.extractability.of.the.respective.ion.pairs.formed.has.increased,.which.is.
well.relected.in.the.growing.magnitude.of.the.end-point.breaks.of.individual.potentiometric.titra-
tion. curves. (the. previous. set).. As. illustrated. by. the. following. “Δ E . vs.. n C ”. plot,. the. phenomenon.
observed.has.exhibited.a.fairly.linear.trend.when.being.related.to.the.number.of.carbon.atoms.in.
the.varied.alkyl.chain,.R.
A.brand.new.experiment.is.then.depicted.in.Figure.6.10,.revealing.that.some.carbon.pastes.may.
manifest.a.notable.activity.in.the.solutions.with.different.pH..Although.these.measurements.have.
primarily. been. focused. on. pH-sensing. with. CPEs. modiied. with. bismuth. [587]. and,. newly,. with.
mixtures. of. Bi. +. Bi 2 O 3 . ([1976];. both. in. powdered. form),. the. initial. studies. with. bare. CPEs. have.
shown.clearly.that.even.carbon.paste.alone.may.act.as.a.pH-electrode.with.limited.operational.range.
Among.the.examples.shown.in.the.graph.and.tested.in.an.interval.of.pH.5-13,.this.was.the.case.
of.a.CPE.made.of.ordinary.graphite.powder.(originally.produced.as.gear.lubricant),.the.respective.
“CR-5/SO”.carbon.paste.exhibiting.fairly.reproducible.and.relatively.stable.response.with.sub-Nernstian.
slope.of.about.40.mV/pH.(full.line)..Compared.with.this,.the.second.“RW-B/SO”.carbon.paste.pre-
pared.from.a.special.spectroscopic.graphite.of.high.grade—and,.in.fact,.representing.“standard”.
carbon. paste. component. [82-84]—has. shown. considerably. lower. activity. in. dependence. of. pH.
(dashed.line),.moreover,.with.a.very.long.time.needed.to.attain.the.steady-state.response.
Such. a. behavior,. together. with. other. comparisons,. has. implied. that. the. observed. pH-activity.
is. associated. with. the. quality. of. the. graphite. used. and. speciically,. with. the. feasibility. of. carbon.
powder.particles.to.have.partially.altered.surface.states.[68],.containing.various.oxygen-containing.
functional. groups. that. can. readily. be. protonized. or. deprotonized. in. dependence. of. pH. (see. also.
Sections.3.2.and.3.4).